Electrostatic reproduction method and apparatus employing a photoemissive surface



01:1. 20, 1910 M PE 3,535,111

ELECTROSTATIC REPRODUCTION 11 HOD AND A RATUS EMPLOYING A PHOTOEM S IVESURFAC Filed y 13, 1966 LWENTOR. M 0 H i n P o p e IQni/, d

757 141001 I I w ATTORNEYS United States Patent 3,535,111 ELECTROSTATICREPRODUCTION METHOD AND APPARATUS EMPLOYING A PHOTOEMISSIVE SURFACEMartin Pope, Brooklyn, N.Y., assignor to Research Corporation, New York,N.Y., a nonprofit corporation of New York Filed May 13, 1966, Ser. No.549,950 Int. Cl. G03g 5/00 US. Cl. 96-1 Claims ABSTRACT OF THEDISCLOSURE A layer of material having an insulated photoemissive surfaceis exposed to a light image of frequency f to cause the externalemission of electrons therefrom. The photoemissive surface hassubstantial adsorption at the frequency f and an external ionizationthreshold B such that hf is less than B and 2 hf is greater than E,where h is Plancks constant. The externally emitted electrons areaccelerated away from the surface on the side thereof away from thelayer to produce an electrostatic latent image which is processed toyield a visible image. Advantageously a negative electrostatic charge isapplied to the photoemissive surface before exposure to the light image.

This invention relates to a method and apparatus for producing anelectrophotographic image, and more particularly to a method andapparatus for producing such image by means of emission of electronsfrom a photoemissive surface.

My invention utilizes a recent and important scientific discoveryconcerning the photoelectric activity of certain materials. Thediscovery has been published in the scientific literature (seeDouble-Quantum External Photoelectric Effect in Organic Crystals by Popeet al., The Journal of Chemical Physics, vol. 42, No. 7, Apr. 1965), andis summarized here. Briefly, it has been found possible to ejectelectrons from suitable insulating materials using high intensity lightof a wavelength much longer than anyone has previously expected would beeffective.

Ordinarily, the ejection of electrons from the surface of an insulatorrequires the use of light of very short wavelength. Typically thewavelength is less than 2500 AU. (angstrom units), in the farultra-violet region. However, I have observed that by illuminatingcertain organic materials with high intensity, strongly absorbed lightof energy several electron volts smaller than the reported externalionization threshold of the materials, an electron emission can beproduced at a rate proportional to the square of the light intensity. Ibelieve that the high intensity of the light gives rise to what may betermed a double-quantum external photoelectric effect whereby twoexcitons produced in the organic material by two absorbed light quantainteract close to the surface of the material to eject an electron fromthe surface.

Electrophotographic processes have been suggested employingphotoemission from insulating materials by ultraviolet light ofwavelength coresponding to the external ionization threshold thereof.The required short wavelength, e.g. 2500 A.U., has seriousdisadvantages. At these wavelengths very few of the commerciallyavailable materials used for photographic film or transparencies willpermit transmission. Thus, a photographic transparency containing animage to be copied would be uniformly opaque at this wavelength andcould not be used to project the image onto a photosensitive surface. Inaddition, at these short wavelengths the reflective power of surfacessuch as white paper becomes seriously compromised in the sense thatcontrast between white and black tends to disappear. Finally, alloptical systems must be made of 3,535,l ll Patented Oct. 20, 1970quartz, since the transmission of ordinary lens materials at thesewavelengths is extremely poor. For these reasons electrophotographicimaging systems using photoemission of electrons from an insulatingsurface have had little if any, significant practicability.

In accordance with the present invention, a photo emissive insulatingsurface is exposed to a light image to cause the emission of electronstherefrom, and the emitted electrons are accelerated away from thesurface by an electric field to produce an electrostatic latent image onthe surface. The material forming said insulating surface and the lightfrequency f are selected with respect to each other so that the materialhas a substantial absorption at the frequency f and an externalionization threshold B such that hf (the energy of a photon) is lessthan B and 2 hf is greater than E where h is Plancks constant.Advantageously, a negative charge is applied to the insulating surfaceprior to exposure to the light image. This greatly improves theefficiency. A high light intensity is employed say of the order of 10quanta/cm. /sec., so that suflicient photoemission is obtained to yielda useable electrostatic latent image.

In this manner, it becomes possible to use wavelength radiation in thenear ultra-violet region (for example, 3930 AU. close to the visibleregion) to produce an electrophotographic image by photoemission,previously obtained only with much shorter wavelengths, thereby escapingthe above-mentioned limitations inherent in the use of shorterwavelengths.

Materials suitable for use in this invention are those in which mobileexcitons are generated by light. Such materials include certainconjugated aromatic hydrocarbons, particularly anthracene, tetracene andperylene.

There are believed to be at least two different types of mobile excitonsthat can coexist in these materials, one of which is generated at alower energy than the other. These types of excitons are called Frenkel,or neutral excitons, and Wannier, or ionic (charge-transfer) excitons.In a given'material the Frenkel exciton generally requires less energyfor its creation than the Wannier exciton. In either case, however, theenergy required to produce the exciton is much less than the reportedexternal ionization threshold for the material.

It has been observed that an external photoelectric emission from amaterial can be produced by what is believed to be the mutualannihilation of two excitons of either type. Thus, in anthracene both anelectron emission attributable to the annihilation of two Frenkelexcitons and an emission attributable to the annihilation of two Wannierexcitons have been observed. In the case of tetracene and peryleneelectron emission is produced by what is believed to be the mutualannihilation of two Wannier excitons.

Materials suitable for use in this invention have, in general, beenfound to fluoresce. Fluorescence is usually a good indication thatFrenkel excitons have been created and that they have a reasonably longlifetime. Single component systems, rather than mixtures or complexesare usually preferable. Moreover, it is desirable to choose materialsthat have a high absorption coefficient in the energy region suitablefor the generation of Frenkel excitons. It is also important that theenergy required to generate excitons in a material be more than half theenergy required to produce an external photoelectric effect.

Different materials will have different external ionization thresholdsand different absorption curves. It is desirable to select a materialhaving a strong absorption peak near the visible light range, and selectthe light frequency to be at or near this peak. Thus anthracene has astrong absorption at 3930 A.U., producing a Frenkel exciton with anenergy of 3:15 electron volts. This exciton energy is more than one-halfof the energy of 5.65 electron 3 volts at 2180 A.U. required to produceexternal photoelectric emission from the material, and satisfies theabove conditions.

Describing a preferred embodiment of the invention in more detail, auniform negative charge is applied to a surface comprised of theabove-described photoemissive insulating material. The surface is thenexposed to a high intensity light image of the subject to be copied orreproduced. Although of high intensity, the quantum energy of the lightimage radiation may be substantially less than the normal work functionenergy of the material, that is, longer wavelength light may be used toform this light image. As already described, the double-quantum processis thus induced in the illuminated surface and electrons are emittedfrom the surface in the areas illuminated by the light image. Anelectric field is provided to accelerate the emitted electrons away fromthe surface, and a latent electrostatic image in the form of the lightimage remains on the surface.

This latent electrostatic image may then be processed in the same Way asin conventional electrophotographic processes. The latent image may bedeveloped into a visible image directly on the surface by theapplication of an electroscopic developer material which is attracted tothe positively charged areas of the latent image and repulsed by thenegatively charged areas. Or, a different electroscopic developer may beapplied which is selectively attracted to the negatively charged imageareas, instead. The resulting visible image may be fixed permanently onthe surface by heat or other means, or it may be transferred to a secondsurface, such as a paper sheet, by pressing it in contact with thesecond surface. The visible image may then be fixed on the secondsurface. In this latter case the photoemissive surface may be cleaned,if necessary, and used again to produce other images.

An important feature of my invention is the initial application of auniform negative charge to the photoemissive surface. Without thenegative charge, the exposure of the surface to the light image wouldstill produce the emission of electrons from the surface. However, theemitting areas would =become increasingly positive as electrons wereemitted, thereby producing a retarding field effect on electronemission. The emission from the surface would therefore be dependent onthe degree of charge residing on the surface, as well as on theintensity of the incident light. Since it is the usual purpose ofphotography to render an image that is faithful to the gradations inintensity of light falling upon a photosensitive surface, this chargedependency is generally undesirable. It is avoided in the presentinvention by charging the photoemissive insulating surface withelectrons prior to exposure to light, preferably to a negative potentialequal or greater in magnitude than the change in potential produced bythe photoemission. In this way there is no retarding field effect andthe photoelectric response is dependent only on the intensity of theincident light.

The invention will be further explained in connection with theaccompanying drawings in which:

FIG. 1 is a schematic drawing of a first embodiment of theelectrophotographic apparatus of the invention; and

FIG. 2 is a schematic drawing of a second embodiment thereof.

Referring to FIG. 1, a sheet of paper 1 is coated with a layer 2 ofanthracene, a material previously noted to be suitable for use in thisapplication. The coated sheet may be moved over an electrically groundedmetal plate 3 in the direction indicated by the arrows. A transparent,electrically-conductive sheet 4, such as NESA coated glass is mountedover the plate 3 and above that is mounted a photographic negative 5bearing an image of the material desired to be copied. A light source 6capable of producing a high intensity burst of light of 3950 A.U. isprovided and mounted above the negative 5.

An electrostatic charger 7, such as a negative corona charger,electroscopic developing means 9' and fixing means 10 are also mountedover the base plate 3. The conductive sheet 4 is connected to thepositive terminal of a DC voltage source, shown diagrammatically as abattery 8, to produce a field between the conductor 4 and plate 3 ofabout 1000 volt/ cm.

In operation the coated sheet of paper is moved on the plate 3 under theenergized charger 7 which applies a uniform negative electrostaticcharge to the exterior sur face of layer 2. The sheet is then movedunder the conductor 4 and the light source '6 is energized to expose thelayer 2 to high intensity 3-950 A.U. radiation which is transmittedthrough the image areas of the negative 5. As explained above, theillumination causes the emission of electrons from the exterior surfaceof the layer 2, and these emitted electrons are accelerated toward, andcollected by, the positive transparent conductor 4. A latentelectrostatic image of the subject to be copied remains behind on thesurface of layer 2.

When the light source 6 is turned oif, the sheet is moved under thedeveloping means 9 where a positively charged dark powder is applied tothe latent image to generate a visible image, since the powder willselectively adhere to those portions of the surface of layer 2 whichwere not exposed to radiation through the negative 5. The sheet may thenbe passed under the fixing means 10, such as a heat source, and thevisible image is made permanent on the sheet.

In the embodiment shown in FIG. 2, a grounded metal drum 11 is arrangedto rotate in the direction of the arrow. The surface of the drum iscoated with a layer 12 of the photoemissive insulating materialspecified above. About the periphery of the drum 11 are mounted anelectrostatic negative charging means 13, a transparent conductive sheet14 conducted to a positive voltage source, electroscopic developer means15 and a drum surface cleaning means 16, such as a rotary brush. A highintensity light source 17 is mounted over the drum so that itsillumination passes through light portions of a photographictransparency 18 of a subject to be copied, and is focused by a lens 19onto the surface 12 of the drum. A web 20 of paper from a reel 21 isbrought in to contact with the drum surface 12 by a roller 22, andfixing means 23 are mounted over the travel path of the Web 20.

In the operation of this embodiment, the drum 11 is rotated in thedirection shown and the charger 13 is energized to coat the surface 12with a layer of negative charge. The drum rotation is then stopped andthe light source 17 is energized to produce, as in the first embodiment,a latent electrostatic image on the surface 12. When the light source 17is turned off the drum is rotated further to bring the latent imageunder the developer means 15, where it is made visible, and then broughtinto contact with and transferred to web 20 by the action of pressureroller 22. As the drum rotates further, the paper web with the visibleimage thereon moves under the fixing means 23 and the image is madepermanent. As the drum continues to rotate, the surface 12 is cleaned bycleaning means 16 to be ready for reproducing another image upon furtherrotation.

It will be understood that various changes in the details, materials,steps and arrangement of parts which have been herein described andillustrated in order to explain the nature of the invention may be madeby those skilled in the art within the principle and scope of theinvention as expressed in the appended claims.

I claim:

1. A method of forming an electrophotographic image which comprisesexposing a layer of material having an insulated photoemissive surfaceto a light image of frequency f for causing the external emission ofelectrons from said surface at the portions thereof exposed to the lightof said light image, said light image impinging on said surface and saidexternal emission of electrons being from the side thereof away fromsaid layer, and applying an external electric field to accelerate theexternally emitted electrons away from said surface on the side thereofaway from said layer to produce an electrostatic latent image on saidsurface, said photoemissive surface having an absorption peak in theregion including said frequency f and an external ionization threshold Bsuch that hf is less than E and 2 hf is greater than E, where h isPlancks constant.

2. The method according to claim 1 including applying a negativeelectrostatic charge to said insulated photoemissive surface prior toexposure thereof to said light image.

3. The method according to claim 2 including developing saidelectrostatic latent image by applying an electroscopic substance to thesurface to form a visible image upon said surface.

4. The method according to claim 3 including fixing the visible imageupon said photoemissive surface.

5. The method according to claim 3 including transferring said visibleimage to a support surface and fixing the visible image thereupon.

6. Electrophotographic apparatus which comprises a layer of materialhaving an insulated photoemissive surface which has an absorption peakin'a region including a frequency f and an external ionization thresholdB such that hf is less than E and 2 hf is greater than E, where h isPlancks constant, electrode means spaced from said photoemissive surfaceon the side thereof away from the layer, means for applying a positivepotential to said electrode means to accelerate externally emittedelectrons away from said surface, and means for exposing said surface toa light image of said frequency f to cause the external emission ofelectrons from said surface in the portions thereof exposed to the lightof said light image and produce an electrostatic latent image on saidsurface, said light image impinging on said surface and said externalemission of electrons being from the side thereof away from said layer.

7. Apparatus according to claim 6 including means for applying anegative electrostatic charge to said insulated photoemissive surfaceprior to exposure thereof to said light image.

8. Apparatus according to claim 7 including means for applying anelectroscopic material to said latent image to form a visible image.

9. Apparatus according to claim -8 including means for fixing saidvisible image on said surface.

10. Apparatus according to claim '8' including means for transferringsaid visible image to a support surface and means for fixing the visibleimage upon the support surface.

References Cited UNITED STATES PATENTS 11/1966 Hoegl 961.5 7/1967 Kosche961.5

U.S. Cl. X.R. 9 6-1.5; 355-3

